(1) Field of the Invention
The present invention relates to a process for producing maleic anhydride, and more particularly, to a process for producing maleic anhydride of improved thermal stability and color stability.
(2) Description of the Related Art
Maleic anhydride is an unsaturated dibasic acid obtainable by the catalytic oxidation of benzene or a fraction of hydrocarbons containing 4 carbon atoms (hereinafter referred to as the C.sub.4 fraction). Maleic anhydride has two carboxyl groups in the form of acid anhydride and a highly reactive double bond so that maleic anhydride is used for various reactions.
Maleic anhydride is highly useful and has been widely used as a raw material for producing synthetic rubbers, plasticizers, synthetic resins, adhesive agents, synthetic fibers, synthetic leathers, agricultural chemicals, and the like.
Maleic anhydride is usually produced by catalytically oxidizing benzene or the C.sub.4 fraction in a vapor phase and distilling the resulting reaction products. However, maleic anhydride obtained by distillation still contains trace amounts of impurities that are difficult to separate such as by-products produced during the oxidation. Therefore, the thus-obtained maleic anhydride lacks thermal stability and has a tendency to become discolored when heat-melted. In particular, this tendency becomes marked when maleic anhydride is stored for a long period of time. This discoloring upon heat-melting exerts adverse effects on the quality of secondary products produced from maleic anhydride and impairs their commercial value to a great extent.
Accordingly, it is strictly required that maleic anhydride for industrial use be free from the discoloration phenomenon upon heating and melting, and during storing for a long time.
In view of the above, various methods have been proposed for the prevention of the discoloration of maleic anhydride.
For example, in Japanese Patent Publication No. 19,405/66 is disclosed a method for improving the thermal stability of maleic anhydride, in which a compound represented by the following general formula: ##STR1## (wherein R.sup.1 represents -H or -OH; and R.sup.2 represents -H, -C(CH.sub.3).sub.3, -COOH or an aliphatic ester thereof) is added to maleic anhydride, optionally together with zinc, copper, aluminum or an oxide or salt thereof such as cuprous chloride, zinc chloride, aluminum chloride and the like. In the method described in Japanese Patent Publication No. 26,766/72, hydroquinone is added to maleic anhydride in combination with elemental copper, molybdenum a copper or molybdenum compound, or a mixture of these, in order to stabilize the color of molten maleic anhydride. In Japanese Patent Publication No. 51,393/82 is disclosed a method for stabilizing the color of maleic anhydride, which comprises incorporating an organic phosphoric polyester, a metal salt of fluoroboric acid, a metallic copper and a copper compound into purified maleic anhydride. In Japanese Patent Application (Laid Open) No. 116,023/74 is described a method in which a phosphorous ester or a mixture of a phosphorous ester and a metal halide is added to maleic anhydride, so as to improve the thermal stability thereof. In Japanese Patent Application (Laid-Open) No. 23,719/73, maleic anhydride is stabilized against discoloration at elevated temperatures through the addition of thiophosphoric hydrocarbinol.
However, none of these prior art methods is able to fully meet the severe requirements in quality of recent years.